280 



STRENGTH OF MATERIALS 



kinds of loads, and providing for the effect of vibration by in- 

 creasing the live-load stress or bending moment by an amount 

 / determined from a so-called impact formula. The formula 

 most in use for railroad bridges is 

 r 300 



L+300 



in which 5 = maximum live-load stress or bending moment in 

 the member, and L = length, in feet, of single track that must 

 be loaded in order to obtain the value 5. 



SIMPLE, OR DIRECT, STRESS 



Formula for Simple Stress. If P is an external force pro- 

 ducing tension, compression, or shear uniformly distributed 

 over an area A, and 5 is the unit working stress, then P = sA 

 is the fundamental formula for designing parts of structures 

 subjected to a simple, or direct, stress. When designing mem- 

 bers that are in tension, A must be taken as the net area of 

 the section. This is determined by deducting from the gross 

 section the greatest number of pin, bolt, or rivet holes that can 

 be cut by a plane at right angles to the section. Rivet holes 

 are usually taken i in. larger than the diameter of the rivet. 



Important Applications of Formulas for Direct Stress. 

 1. Tension members and short compression members of roof 



M 



FIG. 2 



or bridge trusses are examples of simple stress, and their 

 sections are determined by the preceding formula. 



EXAMPLE. A tension member of a roof truss is made of two 

 3}"X3i"Xj" angles connected by one line of rivets fin. in 

 diameter. What stress will it carry at 16,000 Ib. per sq. in.? 



SOLUTION. The gross sectional area of a 3J"X3J"XJ" 

 angle is 3.25 sq. in. The deduction for one rivet hole is (f + 1) 



